The average NFL career lasts just 3.3 years, according to ESPN, and 78 percent of professional football players go broke two years after retirement.

Willie Thomas, a former NFL defensive back who played with the Seattle Seahawks for two years, is part of the 22 percent who has found lasting success off the field. As Managing Director and Market Executive for Merrill Lynch Wealth Management, Thomas focuses on helping today's athletes manage their wealth. The San Francisco-based former athlete tells CNBC Make It that he knows first-hand the financial pressure that athletes face when they first enter the league.

That's why, he says, there's one piece of advice he gives all athletes about how to handle their money.

"Everyone wants to take risks, whether it be on the golf course, at a car dealership, with a restaurant purchase, with jewelry or with the various business deals coming their way," he says. "I just encourage athletes to take calculated risks that are only 3 - 5 percent of their income."

Thomas says the big financial risks he took that he now regrets are his investments in houses and cars. Rather than dropping hundreds of thousands of dollars on a house, he says he wishes he would have looked at the cost benefit of renting, because you never know if you're going to get traded to a new team and have to move.

"The same goes with cars," he says. "You have to figure out how you are going to get one car from, say for instance, Seattle to Florida. Those are things you don't always think about."

He says that when many athletes sign contracts there's pressure not only to splurge on big-ticket items like houses and cars, but also to enter into business deals with family and friends. He explains that "sometimes you have good family members and friends you grew up with and you get this inclination to go all in and invest like 20 - 50 percent of your pay, but I'm a big believer that you shouldn't. We have seen detrimental stories about life savings being gone when a deal went under."

Thomas says athletes should invest in a trusted financial planner early in their career. He advises interviewing three to five people who have years of experience and can help you reach your financial goals.

While Thomas' advice is directed at athletes, it can be applied to other professionals as well.

"The effective outcome that I want for everyone, particularly early NFL players coming into the league with lucrative contracts, is to know about financial wealth planning," says Thomas. "We grow up in a team environment, and so you want to sit around the table with a diverse team that includes a CPA, a sports agent and a financial planner who you're confident in."

Correction: An earlier version misstated Willie Thomas' position on the Seattle Seahawks. He was a defensive back.